The present invention relates generally to heater controls for buildings, such as warehouses or the like and, more particularly, to a heater control which controls the heater such that a temperature rise in the air of the warehouse or building does not exceed a maximum temperature rise value.
Direct fired recirculating industrial air heaters are operable to discharge heated air through ducts within a building or structure, such as a warehouse, storage facility or the like. National Standards mandate that the amount of heat added by the heaters in proportion to the amount of outdoor air being introduced into the building is below a predetermined maximum temperature rise value. More particularly, ANSI Z83.18 mandates control of the amount of heat added in proportion to the amount of outdoor air introduced into the heater. The standard sets the maximum allowable temperature rise based upon the actual percentage of outdoor air being inducted into the heater. The maximum temperature rise is determined by the percentage of outdoor air and a xe2x80x9ckxe2x80x9d factor associated with the type of fuel that is used for heating the building. For example, the k factor for natural gas is 1.04. This requirement is intended to limit the potential for excessive accumulation of carbon dioxide (CO2) within the building.
However, it is difficult to determine the actual percentage of outdoor air being inducted in a precise manner without excessive cost to the air heating system. Flow sensors may be installed in the outdoor and return airstreams and the fraction or percentage of outdoor air being inducted may be calculated based on their measurements. The outdoor and return airstream temperatures can then be measured, whereby the average mixed temperature, using the outdoor air and return air fractions and temperatures, may be calculated and subtracted from the measured discharge temperature. Although this would provide an accurate measurement of outdoor air fraction and temperature rise of air in the heater, it is expensive due to the high cost of flow sensors.
Without flow sensors, it is difficult to determine the precise amount of outdoor air being inducted, and the temperature of the total air being inducted into the heater (i.e. the mixed air temperature), since some heater designs have a small mixing chamber where it is difficult to obtain an accurate measurement. Also, some designs provide more than one outdoor air path, which may adversely affect the accuracy of the temperature sensor of the mixing chamber, depending on the location of the sensor. Additionally, the heater or burner may be provided in one of the air paths of some designs, which further adversely affects the temperature readings in determining the mixing air temperature.
Therefore, there is a need in the art for a low cost system for controlling a heater which heats a building to a desired temperature, while maintaining the rise in temperature in the air in the heater below a maximum temperature rise determined by the percentage of outdoor air being inducted into the heater of the building.
The present invention is intended to provide a low cost control system for controlling a heater or furnace within a building, whereby the temperature rise of the air flowing through the heater is maintained below a maximum temperature rise value. The maximum temperature rise value is determined by a percentage of outdoor air being inducted into the heater of the building, with respect to the return air also being inducted into the heater from within the building. The control of the present invention measures the temperature of the outdoor air and of the return air and determines a percent of outdoor air being inducted into the heater via a position of a damper at the building (through which the outdoor air is inducted into the building) and an efficiency or error rating of the damper. The heater is modulated in response to the temperature sensors and position of the damper.
According to an aspect of the present invention, a recirculating air heater comprises a heater, an outdoor air path and a return air path into the heater, a damper, first and second temperature sensors, and a control. The damper is movable to adjust relative flows through the outdoor air and return air paths. The first temperature sensor is operable to measure outdoor air temperature at the outdoor air path, while the second temperature sensor is operable to measure return air temperature at the return air path. The control is operable to control the burner of the heater in response to the first temperature sensor, the second temperature sensor, and a position of the damper. Preferably, the present invention includes a third temperature sensor at a discharge air path, which measures a discharge air temperature. The control is preferably further responsive to the third temperature sensor. The position of the damper may be determined by a position sensor at the damper.
According to another aspect of the present invention, a method of controlling a recirculating air heater which has a heater or burner, an outdoor air path, a return air path, a discharge air path and a damper, comprises providing a first temperature sensor which measures outdoor air temperature at the outdoor air path. A second temperature sensor is provided which measures return air temperature at the return air path. The damper is movable to adjust the relative flows through the outdoor and return air paths. The control controls or modulates the burner in response to the first temperature sensor, the second temperature sensor, and/or a position of the damper.
Therefore, the present invention provides a heater control which modulates or adjusts an output of the heater to increase the temperature of a building to a desired level, while maintaining a temperature rise of air through the heater at or below a maximum allowable temperature rise, as determined by a percentage of outdoor air being inducted into the heater. The heater control system provides a low cost system, which includes temperature sensors for measuring outdoor air temperature, return air temperature, and the discharge air temperature of the heater. The heater may thus be modulated to maintain an appropriate rise in temperature, without costly flow sensors and the like.